In the field of optical communication, a wavelength-division multiplex transmission system has been studied wherein a plurality of signals are put on light having a plurality of wavelengths with a wavelength spacing of about 1 nm and the light loaded with the plurality of signals are transmitted through one optical fiber to increase communication capacity. In this system, an optical wavelength-division multiplexer/demultiplexer for multiplexing or demultiplexing a plurality of signal lights having a predetermined wavelength spacing plays an important role. Among others, an optical wavelength multiplexer/demultiplexer using an array waveguide diffraction grating can increase the number of multiplexings in communication capacity at a narrow wavelength spacing and, hence, is promising.
The conventional optical wavelength multiplexer/demultiplexer using an array waveguide diffraction grating is disclosed in, for example, Japanese Patent Laid-Open No. 163406/1992 and the like. According to the conventional optical wavelength multiplexer/demultiplexer, a wavelength multiplexed signal light with N waves of wavelengths .lambda..sub.1 -.lambda..sub.N having wavelength spacing .DELTA..lambda. being multiplexed, upon entrance into an input channel waveguide through an optical fiber, is broaden by the diffraction effect in an input slab waveguide and, after branching, enters a plurality of channel waveguides constituting an array waveguide diffraction grating. To the wavelength multiplexed signal light, which has entered the plurality of channel waveguides, is given a phase difference according to the waveguide length of the channel waveguide for each wavelength .lambda..sub.1 -.lambda..sub.N. This permits signal light of .lambda..sub.1 output from the channel waveguide to be entirely condensed in a position corresponding to the incident section of the output channel waveguide. Similarly, signal lights of .lambda..sub.2, .lambda..sub.3 . . . .lambda..sub.N are condensed in positions corresponding respectively to incident sections of the output channel waveguides. The signal lights, which have been demultiplexed and condensed in the incident sections of the output channel waveguides, are propagated through the output channel waveguides and output from the output side thereof.
According to the conventional optical wavelength multiplexer/demultiplexer, there is a difference in density of the plurality of channel waveguides between the center portion and both end portions. Therefore, in the preparation of a plurality of channel waveguides, even when the cores are designed to have an identical width, the difference in etching speed unfavorably creates a variation or fluctuation in core width. The phase velocity of the array waveguide diffraction grating depends upon the core width, and the variation and fluctuation in core width create a deviation from the set value with respect to the condensing position and the amount of condensation within the output slab waveguide.
On the other hand, the creation of a deviation from the design value with respect to the phase difference of the channel waveguides and the creation of a deviation of the condensing position of the signal light results in the creation of tailing. The tail of the signal light is superposed on a waveguide mode of the different output channel waveguide, providing such a wavelength loss curve that a very small amount of light is present in a wavelength distant from the wavelength .lambda..sub.i. This imperfect condensing is causative of increased light leakage to a different service band, posing a problem of deteriorated crosstalk.